docs: def _iso_ic_on_planck(
          electron_energy, soft_photon_temperature, gamma_energy
      ):
          """
          IC cross-section for isotropic interaction with a blackbody photon
          spectrum following Eq. 14 of Khangulyan, Aharonian, and Kelner 2014,
          ApJ 783, 100 (`arXiv:1310.7971 <http://www.arxiv.org/abs/1310.7971>`_).
  
          `electron_energy` and `gamma_energy` are in units of m_ec^2
          `soft_photon_temperature` is in units of K
          """
          Ktomec2 = 1.6863699549e-10
          soft_photon_temperature *= Ktomec2
  
          gamma_energy = np.vstack(gamma_energy)
          # Parameters from Eqs 26, 27
          a3 = [0.606, 0.443, 1.481, 0.540, 0.319]
          a4 = [0.461, 0.726, 1.457, 0.382, 6.620]
          z = gamma_energy / electron_energy
          x = z / (1 - z) / (4.0 * electron_energy * soft_photon_temperature)
          # Eq. 14
          cross_section = z ** 2 / (2 * (1 - z)) * G34(x, a3) + G34(x, a4)
          tmp = (soft_photon_temperature / electron_energy) ** 2
          # r0 = (e**2 / m_e / c**2).to('cm')
          # (2 * r0 ** 2 * m_e ** 3 * c ** 4 / (pi * hbar ** 3)).cgs
          tmp *= 2.6318735743809104e16
          cross_section = tmp * cross_section
          cc = (gamma_energy < electron_energy) * (electron_energy > 1)
          return np.where(cc, cross_section, np.zeros_like(cross_section))